Patent Literature 1: JP 2005-168047 A
There is conventionally known a communication apparatus that communicates with an external apparatus by generating communication signals according to a PAM (Pulse Amplitude Modulation) mode. One of PAM modes is PMA5 that encodes informational signals into five levels of values. Demodulating is performed by detecting the corresponding voltage levels of the reception signals on the basis of a period of each symbol.
PAM5 mode is adopted in 100BASE-T2 that is a standard of 100-megabit Ethernet (registered trademark) or 1000BASE-T that is a standard of gigabit Ethernet. According to 100BASE-T2, a 4-bit data is encoded into two PAM signals each of which indicates five different levels of values, and the two PAM signals are transmitted to an external apparatus via the respective two transmission lines.
In addition, 4D-PAM5 mode is adopted in 1000BASE-T. According to 1000BASE-T, an 8-bit data is encoded into four PAM signals each of which indicates five different levels of values, and the four PAM signals are transmitted to an external apparatus via the respective four transmission lines.
Another PAM mode is PAM3/4/16 mode that encodes informational signals into 3/4/16 different levels of values. PAM16 mode is adopted in 10GBASE-T that is a standard of 10 gigabit Ethernet.
In addition, in order to reduce inter-symbol interference (ISI) or electromagnetic interference (EMI), a signal transmission according to PAM modes transmits PAM signals into a transmission line via a pulse shaping filter. According to 100BASE-T2, for example, the pulse shaping filter whose transfer function is H(z)=0.75+0.25z^−1 (i.e., H(z)=0.75+0.25z−1) permits pulse shaping of PAM signals from the physical coding sublayer; the PAM signals after the pulse shaping are converted into the analog signals through a digital-analog converter. The analog signals are sent out into the transmission line via a hybrid circuit (refer to Patent Literature 1).
Incidentally, the bit number (gradation number) of a digital-analog converter located in the latter stage of a pulse shaping filter affects the magnitude of the circuit. According to a pulse shaping filter of which the number of taps is 2, an output value is determined by a combination of an input value at the present time and an input value at the previous time. Therefore, when PAM signals according to PAM5 mode are inputted into the pulse shaping filter, the output values from the pulse shaping filter become 5×5=25 kinds at the maximum. It is noted that the output values become an identical kind depending on the combination of input values. In this case, the output values of the pulse shaping filter whose transfer function is H(z)=0.75+0.25z^−1 indicate 17 kinds of values.
When the output values of the pulse shaping filter provide 17 kinds, the bit number (gradation number) of the digital-analog converter necessary for the latter stage of the pulse shaping filter is equal to or greater than 5 bits, which is still the same as that when the output values of the pulse shaping filter provide 25 kinds. In addition, the pulse shaping filter using the above transfer function H(z) still has room for improvement in the respect of the reduction effect of EMI in pulse shaping.